Question 6 of 7: Thermoplastics vs. Thermosets, Polymer Additives, and Factors Affecting Plastic Properties

Question 6: Thermoplastics vs. Thermosets, Polymer Additives, and Factors Affecting Plastic Properties (20 marks: 8/6/6)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(i) Thermoplastics vs. Thermosetting Plastics

Plastics divide into two fundamental groups based on molecular structure and how they respond to heat:

• Thermoplastics consist of linear or branched polymer chains held together only by secondary (van der Waals/intermolecular) bonds. Heating breaks down these weak secondary bonds so the material softens and flows, and it re-hardens on cooling; because no permanent chemical (covalent) bond is formed or broken, this heating/cooling cycle can be repeated many times, making thermoplastics recyclable and well suited to processes like injection molding and extrusion. They are generally tougher and more ductile than thermosets, but have lower thermal and creep resistance.
• Thermosetting plastics begin as a low-molecular-weight liquid or semi-solid resin that, on heating (or with a curing agent), undergoes an irreversible chemical reaction forming a rigid, three-dimensional cross-linked (covalently bonded) network. Once cured, the cross-links cannot be broken by reheating — the material chars or decomposes rather than melting — so thermosets are not recyclable by remelting. They generally offer higher rigidity, hardness, dimensional stability and heat/chemical resistance than thermoplastics, at the cost of brittleness and a one-shot (non-reversible) forming process.

(ii) Purpose of Additives in Polymers

Additives are compounded into a base polymer resin to tailor its processing behaviour and in-service properties without changing to a different (and often more expensive) base polymer: plasticizers increase chain mobility to improve flexibility and lower processing temperature (e.g. flexible PVC); fillers and reinforcements (calcium carbonate, glass or carbon fibre, talc) reduce cost and/or increase stiffness, strength and dimensional stability; stabilizers (UV, heat and antioxidant stabilizers) protect the polymer from degradation by sunlight, processing heat and oxidation over its service life; colourants (dyes, pigments) provide the required appearance; flame retardants reduce flammability for code-driven applications; and lubricants and processing aids reduce friction against mold/die surfaces and improve flow during molding or extrusion. In every case the additive lets the resin manufacturer or processor hit a target combination of processability, mechanical performance, appearance and cost that the base polymer alone cannot achieve.

(iii) How Various Factors Affect the General Properties of Plastics

The properties of a finished plastic part are governed by several interacting factors: molecular weight and its distribution — higher molecular weight generally raises strength, toughness and melt viscosity (making processing harder); degree of crystallinity — semicrystalline polymers (e.g. polyethylene, nylon) are stronger, stiffer and more chemically resistant but more opaque and prone to shrinkage than amorphous polymers (e.g. polystyrene, polycarbonate), which are typically clearer and more dimensionally stable as-molded; degree of cross-linking — as noted in part (i), cross-linking trades flexibility and recyclability for rigidity and heat/chemical resistance; temperature — properties are strongly temperature-dependent, softening progressively as the glass-transition temperature ($T_g$) is approached and, for thermoplastics, melting outright at the melt temperature; and processing history — cooling rate, orientation of the polymer chains induced by flow during molding or extrusion, and residual stress from processing all affect the final strength, shrinkage and warpage of the part, so two parts of the identical base resin can have measurably different properties depending on how they were processed.